In recent years, electronic computers and other integrated circuit products have had an increasing tendency towards higher operating speeds. As operating speed increases, the problems of noise within the circuitry and malfunctions attributable to the noise become more serious. Principal sources of such noise are firstly the fluctuation of power source voltage, attributed to switching, and secondly the interaction between signal paths i.e. crosstalk.
Any wiring line in a circuit has an inductive component. The size of voltage fluctuation, i.e. noise, caused by this inductive component is proportional to the frequency at which the line is operating. Thus, at high operational frequencies, the inductive contribution of the wiring lines becomes a significant contribution to noise. This is particularly so in a fast-operating electronic computer, in which the operating voltage is generally low; since the induced noise voltages based on induction are not dependent on the power source voltage but on the frequency, their size becomes relatively large and causes a problem.
It is well known to reduce noise due to e.g. power source voltage fluctuation, by inserting a capacitor appropriately in the power source path between the power source and Ground. Usually this is called a "bypass capacitor".
It is known that, to be effective, a bypass capacitor should be positioned as close as possible to the noise Generating source, since the wiring line between the capacitor and the source can give rise to further inductive noise. In an electronic computer, particular noise generation sources are the switching portions or "gates" which lie at the heart of the computer. The operation of these gates Generates noise, and this noise at the same time tends to cause malfunction of neighbouring gates. Accordingly, it is important to install a bypass capacitor close to these switching portions, e.g. a logic LSI, a RAM module, a VRLSI, or a DRAM (dynamic random access memory) in a computer.